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Arsenic contamination in the groundwater of Northeastern India: Critical
understandings on geotectonic controls and the need for intervention
Nikita Neog, Ritusmita Goswami, Durga Prasad Panday, Abhay Kumar, M. Tamil
Selvan, Annapurna Baruah, Manish Kumar
PII: S2468-5844(24)00009-6
DOI: https://doi.org/10.1016/j.coesh.2024.100539
Reference: COESH 100539
To appear in: Current Opinion in Environmental Science & Health
Received Date: 24 May 2023
Revised Date: 6 February 2024
Accepted Date: 6 February 2024
Please cite this article as: Neog N, Goswami R, Panday DP, Kumar A, Selvan MT, Baruah A, Kumar M,
Arsenic contamination in the groundwater of Northeastern India: Critical understandings on geotectonic
controls and the need for intervention, Current Opinion in Environmental Science & Health, https://
doi.org/10.1016/j.coesh.2024.100539.
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Graphical Abstract
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Arsenic contamination in the groundwater of Northeastern India:
Critical understandings on geotectonic controls and the need for
intervention
Nikita Neoga, Ritusmita Goswamia,*, Durga Prasad Pandayb, Abhay Kumarc, M. Tamil
Selvand, Annapurna Baruahb, Manish Kumarb,e, ,*
aCentre for Ecology, Environment and Sustainable Development, Tata Institute of Social
Sciences, Guwahati- 781013, Assam, India
bSchool of Advanced Engineering, UPES, Dehradun-248007, Uttarakhand, India
cCIET, NCERT, Sri Aurobindo Marg, New Delhi – 110016, India
dCentre for the Study of Regional Development, SSS, JNU, New Delhi – 110067, India
eEscuela de Ingeniería y Ciencias, Tecnologico de Monterrey, Campus Monterey, Monterrey
64849, Nuevo Leon, Mexico
*Corresponding author:
Ritusmita Goswami E-mail address: ritusmita100@gmail.com
Manish Kumar E-mail address: manish.env@gmail.com; manish@tec.mx
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Abstract
We investigated possible hypothesis related to arsenic (As) enrichment in the aquifer system
of north-east India, and concluded three governing deductions with explicit evidences,
namely: i) the tropical environment facilitated leaching driven As release from the rocks; duly
supported by reductive dissolution of clay deposition attributed to the seasonal floods in the
region. ii) As-containing quaternary sediments succoured by prevailing plate tectonics in
active convergent tectonic settings of the Eastern Himalayas. and; iii) High precipitation,
tropical climate, and biodiversity richness driven microbial mediated weathering of mineral-
rich rock formations, contributing to As enrichment. We emphasized the uniqueness of the
aquifer systems of north-eastern India for understanding the dynamics of prevalence and co-
contaminations of geogenic contaminations like As-F-U, especially in the context of Karst
aquifer, and surface-groundwater interactions, implying the presence of immense
opportunities of getting several insights on the fate and transport of geogenic contamination
enabling the global management.
Keywords: Plate tectonics; geological investigation; arsenic mobilization; Brahmaputra basin;
Indus-Tsangpo suture zone
1. Introduction
Around 500 million people worldwide are impacted by groundwater arsenic (As) poisoning
[1]. More than 70 countries worldwide have documented natural As pollution in their drinking
water sources, most in South and Southeast Asia, including India, Vietnam, China,
Bangladesh, and Nepal [2]. Arsenic is highly toxic and can cause adverse health consequences
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even at low exposure levels (<10 µg L-1). Groundwater contamination by As is among today's
most prominent and hazardous environmental issues, especially in developing nations like
India. Long-term ingestion of elevated concentrations of As in groundwater-sourced drinking
water affects around 90 million people in India [3]. India is the largest user of groundwater,
and As toxicity dates back to 1978 in West Bengal [1]. West Bengal, Bihar, Uttar Pradesh,
Assam, and Punjab have the highest populations exposed to elevated concentrations of As
[4]. An estimated 9 million people are exposed to groundwater As contamination in Bihar
after its first report in 2002 [1].
In northeastern India, understanding the prevalence of As contamination is crucial for water
resource management, guiding the development of effective treatment technologies and
strategies to remove As from drinking water sources. Research on As toxicity can drive
innovation in water quality testing, treatment technologies, and health interventions, leading
to cost-effective and sustainable solutions. The northeastern region of India, part of the
Brahmaputra floodplains, is a significant groundwater source for millions in the South and
Southeast Asian Arsenic Belt [4, 5]. The source of As in the GBM plain is the Himalayan
Mountain region. In Assam, Manipur, Arunachal Pradesh, Nagaland, and Tripura, the
groundwater has alarmingly high As concentrations (beyond the WHO acceptable limit of 10
µg L-1) [ 6, 7, 8] (Table 1).
Geological, hydrological, and anthropogenic factors can interact in a complicated way to
affect the amount of As in groundwater. Most As-prone zones occur in sedimentary basins
near present-day mountainous regions and deltaic basins [5]. Arsenic degrades groundwater
quality, primarily released from rocks into the water in aquifer systems [10, 11, 12]. Because
a tropical environment encourages the release of As, such regions are more susceptible to As
contamination [1]. Hydrological, biological, and geochemical processes in groundwater
govern the mobilization and distribution of As, leading to spatial variation in groundwater As
levels [13, 14]. Desorptive release of As from iron (Fe)-hydroxide/oxyhydroxides influenced
by microbial activities is the most prevalent mechanism for As release in several As-
contaminated parts of the Indian northeastern region [10, 11, 12, 13, 15, 16, 17]. Oxidizing
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and reducing environmental conditions facilitate As release from As-containing minerals
elemental As, arsenites, arsenides, sulfides, metal oxides, and hydroxides.
Nonetheless, the distribution of this geogenic pollutant can vary geographically due to
differences in parameters affecting its release. Elevated concentrations of geogenic
contaminants besides As, such as Fe, fluoride (F-), lead (Pb), and uranium (U), have also been
reported from different parts of the region [18]. The northeastern region has a distinctive
geospatial and geoscientific setup, with the Indo-Burman orogenic belt traversing Assam,
Nagaland, Mizoram, Tripura, and portions of Arunachal Pradesh. Plate tectonics plays a
pivotal role in As contamination in the northeastern region of India due to ophiolitic rocks,
red radiolarite, and pelagic sediments along the arc [19]. Since Northeastern Hill states were
formed late in the Himalayan orogeny, groundwater As contamination is expected in the flood
plains of these states [22, 23].
Arsenic, identified as a common geogenic contaminant, and its ubiquity and health effects
cause widespread concern [11, 16, 20]. The plight of As contamination in Bangladesh and
West Bengal, which are a part of the larger Bengal basin, is considered the "largest mass
poisoning in human history" [19]. According to the WHO (2011) [21], the economic strain on
affected areas from As toxicity in northeastern India will soon become apparent. This study
presents a complete account of groundwater As prevalent in the northeast region of India. A
comprehensive study shows that apart from Assam, very little knowledge or research has
been done for the other northeast states. Even today, a cavity of detailed analysis exists to
evaluate groundwater quality based on probable As toxicity in the northeastern states. This
review underlines the urgency and significance of studying the same. This study may help
develop plans to evaluate the groundwater quality using a combination of geology and other
environmental indicators. Thereby, the objectives of this research are to provide a
comprehensive review of the As-contaminated regions of northeastern India (>10 µg L-1),
identify As mobilization mechanisms relevant to the study region, and investigate the
association of local environmental, geological, and tectonic factors influencing As availability.
2. The Arsenic Scenario in Northeastern India
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Initially, As-contaminated groundwater was uncovered in Assam's Nalbari, Jorhat, Barpeta,
Dhemaji, Golaghat, Darrang, Nagaon, Sonitpur, Dhuburi, Lakhimpur, Cachar, Hailakandi,
Karimganj, Goalpara, Kamrup, Sibsagar, Dibrugarh, Bongaigaon, Kokrajhar, and Tinsukia
districts [4]. Arsenic in the 100-200 µg L-1 range was found in Dhemaji and Golaghat districts,
while 194-657 µg L-1 was found in Jorhat [5, 24]. High As concentrations (468 µg L-1) are
present in Majuli, Brahmaputra's largest river island [15].
Arsenic concentrations ranged from 10.1-93.05 µg L-1 in the North Karimganj block of the
Karimganj district, and elevated Fe concentrations (2.4-45.3 mg L-1) were also observed [25].
A similar trend was observed in the Nagaon district, Assam [26]. The relationship between As
and Fe suggests that the source of As in the Nagaon district results from the reductive
breakdown of solid Fe oxide and hydroxide phases. According to Kumar et al., As may be
linked to the carbonate phase and the groundwater pH level [25]. In Lakhimpur, amorphous
Fe (hydr) oxide predominates as the principal source of As during the post-monsoon due to
the more anoxic conditions [11, 15]. The presence of As in the groundwater of Majuli is
associated with various factors such as the local aquifer conditions, source rock, and
hydrogeology [16, 18]. According to reports for other areas of the Brahmaputra floodplain,
concentrations of As and other elements are higher at shallow depths. In Majuli, the
population was at increased risk of developing cancer, especially the children who are more
susceptible owing to their lower body weight and a higher level of exposure [17].
High concentrations (798-986 µg L-1) of As were first detected in the groundwaters of Thoubal
district, Manipur [4], later reported by Chakraborti et al., 2013 [23] and Alam et al., 2019 [8].
The analysis of water quality indices for potable drinking water in the Bishnupur district of
Manipur was ranked poor due to geogenic contaminants and other chemical constituents.
Arsenic concentrations ranging > 50 µg L-1 [26] existed in the aquifers of the region at a depth
range of 30-70 m bgl [22, 27]. In some areas in the Bishnupur district, phosphorus
concentrations were higher than recommended (0.1 mg L-1). A significant association
between As and phosphate suggests that anthropogenic agriculture inputs influence As
mobilization in the region [27]. Imphal East and West also reported As in part at ranges of 3-
>500 µg L-1 [22]. The groundwater of the Manipur region is rich in geogenic As [22], mainly
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attributed to the reductive microbial dissolution of Fe-oxyhydroxide leading to the
subsequent release of the sorbed As [28].
Dibang, West Kameng, East Kameng, Lower Subansiri, and Tirap districts in Arunachal Pradesh
also reported groundwater As upto 618 µg L-1 [6, 7]. A recent study in Arunachal Pradesh by
Goswami et al., [29] revealed As-enriched sediments in the region. The state is known for its
coal reserves and has several mines across the state (Arunachal_pradesh - INDIA WRIS WIKI,
n.d.). As-enriched sediments are said to be associated with coals (Pollutants, 1977), which
may eventually find their way to the groundwater. In the state of Tripura, West Tripura
(Triania Block), Dhalai (Salema Block), and North Tripura (Dharmanagar Block) were reported
to have As contaminated groundwater having a concentration in the range of 65-444 µg L-1
[6, 7]. In Nagaland, the districts of Mokokchung, Mon, Wokha, and Zunheboto also have
remnants of As concentrations varying from 50 to 278 µg L-1 [6, 7]. This region is geologically
composed of tertiary-age semi-consolidated deposits and the Tipam, Barail, and Disang
formations [30]. Shales constitute a significant part of the Disang group of rocks, siltstones,
and sandstones. Similarly, the Barail formations also contain shales with sandstone and coal
bands [30]. Hence, by conducting detailed hydrogeochemical studies, researchers can identify
specific geological formations that may contribute to As presence in this region.
Figure 1 depicts the As hazard scenario, associated geology and iron (Fe) contamination in the
northeastern region of India. These are regions where As levels are dangerously high,
potentially posing health risks to the population. Moreover, it shows the distribution of
different geological formations across the region, dominated by quaternary sediments. They
contain naturally occurring As-rich minerals. Quaternary sediments in northeast India play a
critical role in controlling the presence and mobility of As in groundwater. Additionally, Fe
minerals interact with As, affecting its mobility and bioavailability in groundwater.
3. Arsenic Mobilization Mechanisms in Northeastern India
Several factors contribute to the release and transport of As, including geological,
hydrological, and anthropogenic influences. The metal oxide-mediated release of As is a
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complex process that significantly affects the mobility and bioavailability of As in the
environment. Metal oxides, such as Fe and manganese (Mn) oxides, are standard components
of soil and sediment. Arsenic adsorption and desorption from metal oxide surfaces determine
its availability. Further, microbes can either enhance or mitigate As mobility, depending on
the specific microbial activities, such as driving redox reactions and releasing organic
compounds that facilitate the dissolution of As-bearing minerals.
3.1. Metal oxide-mediated release
In North-East India, reductive dissolution of ferric hydroxides appears to be the mechanism
that influences As mobility. The dissociation of Fe from its Fe-hydroxides and ferromanganese
compounds is controlled by reducing conditions aided by microbial activities that mobilize As
into the aquifers. Arsenic is adsorbed onto the ferrous oxides or oxyhydroxides and released
into the aquifers [31].
The weathering of sulphide linked to carbonaceous materials is believed to have formed Fe
oxyhydroxides rich in As, which subsequently releases As, following reduction, to the alluvial
environment [31]. Arsenian pyrite [(Fe(SAs)2] is a significant mineral source of As in sediments
and can release As into the environment through oxidation in aerobic conditions and
adsorption to metal oxides in reducing requirements. It is considered the most common As-
mobilization mechanism in the study region [10, 12, 13, 15, 16, 17, 31, 32] because of Fe-rich
minerals throughout the region [Fig 1]. As reported in our previous study, As-fractionation of
the sediments of Subansiri, Dikrong, and Ranganadi (Brahmaputra River tributaries) revealed
Fe-hydroxides to be significant contributors to As mobilization [13]. The release of As through
pyrite oxidation is as follows in Eq(i) [33]:
…Eq (i)
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Inorganic As in groundwater can be found in different oxidation states depending on the
aquifer's pH and ORP [14, 34]. Redox-sensitive surfaces have shown the ability to oxidize As
(III) to As (V), including those of clay minerals, Fe and Mn (hydro)oxide surfaces, and sulfates.
However, As (V) can be quickly reduced by H2S in highly reducing circumstances, and the
reduction rate increases as the pH decreases [35]. An extensive community of anaerobic
microorganisms, including methanogens, fermentative bacteria, and sulphate- and iron-
reducers, have been found to mediate the reduction of As (V) to As (III) under anaerobic
environments [35]. The groundwater samples of the study area were detected as slightly
acidic [4], and the release of As with organic matter induced bacterial decomposition and
anoxic conditions as shown in Eq-(ii) [36]. The reductive dissolution process of Fe-oxide is
represented by [33]:
--- Eq (ii)
The following chemical reactions Eq (iii & iv) have elaborated the most common pathways of
As release [14].
--- Eq (iii)
--- Eq (iv)
Lysinibacillus sphaericus, Acinetobacter nosocomialis, Pseudomonas aeruginosa, and Bacillus
licheniformis are indigenous microbial species acknowledged as having significant tolerance
of As along with their ability to increase the dissolved arsenate [As(V)] concentration in the
alluvial floodplains of the Brahmaputra [12].
3.2. Role of microbes in arsenic release
Arsenic leaching in groundwater is caused by microbiological processes that reduce Fe and
oxidize sulphates [13, 14, 25, 37] [Fig 2]. Arsenite-oxidizing bacteria are categorized based on
diversity and adaptability [38]. Certain bacterial strains are known for controlling As
mobilization in the subsurface environment through dissimilatory reduction or detoxifying
mechanisms [13, 37, 39, 40]. Figure 2 gives a visual representation of the mobilization and
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transformation of As, which is influenced or facilitated by the presence of certain minerals
rich in Fe, Mn, and S like Pyrite, Pyrolusite, Rhodochrosite and others. These minerals interact
with As and can either immobilize it or make it more mobile in the environment, depending
on environmental conditions. Bacterial growth and arsenate acting as a terminal electron
acceptor are coupled by the dissimilatory reduction ability of bacteria, which occurs in
facultative anaerobic aquifer conditions [13]. This ability where bacteria can use As (V) as an
alternative electron acceptor to oxygen allows microbes to grow in anaerobic environment of
aquifers, sediments, or contaminated sites. Further, as a result of reduction of As (V) to As
(III), these microbes can gain energy yet release less toxic form of As in the groundwater.
Bacteria with the ars and arr operon reduce As (V) in the cytoplasm and extrude As (III)
through the cell wall, causing As to be mobilized via sediment into the aquifer [13, 41, 42, 43].
The alluvial floodplains of the Brahmaputra River host aquifers with reducing conditions that
favour reductive microbial dissolution of Fe and As mobilization [13]. However, research on
the effect of microbiology on As mobilization in the Brahmaputra alluvial plains is scarce.
4. Influence of local geological conditions on arsenic enrichment
Geologically, NER India exhibits a stratigraphic sequence spanning from the pre-Cambrian era
to the Quaternary period. A simplified stratigraphic table (table 1 and table 2) highlights the
major geological periods and formations in North East India [49, 51, 52]. The oldest pre-
Cambrian gneissic complex found in the Meghalaya plateau, and Karbi-Anglong plateau [50,
51]. The Himalayas encompass rock formations of the Protozoic to the early Palaeozoic era [
51, 52]. Tertiary rocks, primarily Mio-Pliocene deposits, forming the foothill zone of the
Himalayan mountains post the orogenic phase. The remaining part of the region is
characterized by Tertiary rocks displaying diverse marine facies, spanning from the Eocene to
Pliocene periods [ 51, 52]. The crystalline rocks, sedimentary rocks, ophiolite belts, and
volcanic formation with the diverse minerals and elements in the region under went
metamorphic and weathering processes [51, 52], implying the Himalayan rocks and the Indo-
Burman ranges serving as the ultimate sources of As. Minerals like biotite, magnetite,
ilmenite, olivine, pyroxene, and amphiboles contain As (table 2) [53, 54, 55, 56, 57]. Arsenic
released from these minerals gets absorbed by secondary minerals, notably iron hydroxides
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like goethite [57, 58, 59]. Under oxidizing conditions, As remains sequestered in these iron
hydroxides, immobilized within the sediments [46, 58, 59].
Previous literatures [60, 61, 62, 63] have reported that the global climate and sea level
transitions during Pleistocene to the Holocene played a crucial role in shaping the distribution
of As in North East India. During the Pleistocene, the earth experienced multiple glacial-
interglacial cycles [64]. Lower sea levels during glaciations led to the exposure of large land
areas, resulted in lower intensity of reduced weathering and erosion due to colder and drier
conditions, possibly limiting the release of As into groundwater [62, 63]. As the earth
transitioned into the Holocene, a warmer and wetter climate prevailed, and increased the
weathering of geological formations [65]. Increased rainfall and higher temperatures
enhanced chemical weathering [65, 66], particularly in areas with specific geological
formations or rock types enriched with As compositions, such as the Indus-Tsangpo suture
zone and Siwalik foreland basin. The process of weathering serpentinites in the Indus-Tsangpo
suture zone leads changes in mineral compositions and, consequently, the release of
elements like As into water sources [46].
Table 1: Maximum values As and Fe reported in the northeastern Indian region and local
geology. Quaternary sediments are the most dominant geological conditions, and Assam
shows very high co-occurrence concentrations of As and Fe.
The Pre-Cambrian gneissic complex of the Meghalaya plateau, a craton, and the Karbi-
Anglong plateau, once part of Gondwanaland, constitute the earliest geological formations in
the northeastern region [44, 45]. The abrupt uplift of Siwalik sediments and increasing runoff
conditions have mobilized the As in the clay layers in the area. Sediment deposits in the
Ganges-Brahmaputra River system in northeastern India can include up to 490 mg Kg-1 of As,
with an average of 95.1 mg Kg-1 in the carbonaceous matter [5, 46]. Weathering of sulphides
in certain carbonaceous materials forms As-rich iron oxyhydroxide, which releases As, under
anoxic environmental conditions into the sedimentary layers of Arunachal Pradesh, Assam,
Nagaland, and Meghalaya [5]. Seasonal flooding of rivers in the Ganges-Meghna-
Brahmaputra River system affects sediment erosion and deposition, texture, surface area,
particle assemblage, clay and sand content, organic matter content, and other elements
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which affect As retention and leaching, making it a significant geogenic contaminant in the
flood plains of the Brahmaputra [5, 13, 32, 47, 48, 49, 69].
Table 2: Major geological periods and a generalized details geological formations with respect
to Arsenic Contamination in North East India [ 51, 52, 53, 54, 55, 56, 57, 70].
Alluvial aquifers in India make up 90% of the country's water sources. The high rainfall in the
Holocene period has increased the weathering of Himalayan rocks, leading to an increase in
finer Fe-hydroxide minerals and fine grained debris in the northeastern region of India.
Numerous studies have linked high As concentrations to dissolved Fe levels and reduced
environmental conditions of aquifers [ 3, 8, 9, 10, 12, 14, 25, 31, 32], as well as to the presence
of microorganisms [12, 37]. The environment, local climate, and season all have a role in the
co-occurrence [14, 71, 72, 73]. The As release has not been extensively studied in
northeastern India, with only a few old cases reported from Nagaland, Arunachal Pradesh,
and Tripura; their hydrogeochemical studies are limited. Thus, no particular mechanism of As
occurrence is known to date.
5. Influence of plate tectonics in geogenic arsenic of Brahmaputra plains
The Himalayan foothills, Indo-Burman Ranges is under intense tectonic activities, which lead
to the exposure and weathering of the minerals, releasing As into the environment [65, 66,
67]. The subduction zones, characteristic of the Indo-Burman Ranges, can generate
hydrothermal systems, which facilitate the movement of mineral-rich fluids, potentially
releasing As into the surrounding geological formations and groundwater [68]. Tectonic
activities influence the redox conditions within aquifers. Changes in these conditions, such as
the presence of reducing environments due to organic-rich sediments or tectonic structures
altering groundwater flow paths, can mobilize As from mineral sources [68]. Groundwater in
reservoirs can potentially contain As based on the typical continental crust content. However,
the most significant cases of groundwater As-enriched regions worldwide tend to be found in
clastic basins near the young mountain range in active or historic convergent tectonic settings
[74, 75]. Mukherjee et al. (2019) [75] suggest As originates from continental convergent
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magmatic arcs, which deposits in adjoining reservoirs containing groundwater. Biological,
tectonic, and geochemical processes can create As-rich mineral sources [74]. Understanding
these geological dynamics is crucial for assessing and managing As contamination, guiding
strategies for safe water resource management in the Brahmaputra plains.
Despite the typically low As content (2000 µg Kg-1) of crystalline rocks and sediments in the
Himalayan units, two zones have the potential to be As-rich due to their abundance of arc-
related rocks and clay minerals enriched in As: the Indus-Tsangpo suture zone and the Siwalik
Neogene sediments [44]. The Indus-Tsangpo suture zone is a significant source of As due to
its arc-related rocks and high volume of As-enriched serpentinites. The Siwalik foreland basin
has served as an As reservoir from the Miocene to the Pleistocene due to the weathering of
rocks in the suture zone [44]. The weathering of serpentinites in the Indus-Tsangpo suture
zone results in the dissolution of Mg and Si and causes a substantial increase of Fe and Al,
forming smectite, kaolinite, chlorite, and oxy-hydroxide minerals, which contain 76 x10-6 µg
L-1 of As [44]. Nonetheless, the Greater Himalayas can be considered the source of As [74].
The formation of the Brahmaputra River was caused by geological folding, faulting, thrusting,
and intrusions, which led to an average of 2760 µg L-1 As deposition in the aquifers [44].
6. Future Approaches
The northeastern region of India is home to 45,161,611 people [73], the majority of whom
rely on groundwater for drinking, irrigation, and other purposes, making them susceptible to
the health hazards posed by As. If the present scenario continues, the northeastern states will
face health and socio-economic hazards, as in Bangladesh and West Bengal. Collaboration
between government agencies, research institutions, and communities has led to the
developing of innovative and cost-effective As remediation technologies in Assam.
Conventional water treatment methods like coagulation, flocculation, sedimentation, and
filtration have been employed to remove As at the household or community level to treat
drinking water. However, further research on how these projects can be implemented on
large scales is essential.
Conventional water resources such as lakes, ponds, and rivers should be analyzed to evaluate
the contamination status. These can be used as alternative sources of contaminated
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groundwater for consumption and irrigation. Precipitation levels of the entire northeastern
region are very high, increasing its potential to harvest rainwater. Stakeholders can promote
watershed management to minimise the use of As contaminated water. Increasing public
knowledge, training on water resource management, and promoting the use of surface water
can help reduce these risks, but early detection of affected aquifers and research on As
sources are also needed to facilitate hazard mapping and prevention. Researchers are still
uncovering the factors that cause the release of As into groundwater. Further detailed
studies, such as understanding the decadal shift in the As groundwater pattern and the AI-
based approach for predicting future contamination, are crucial. Moreover, the citizen
science approach, which will educate people to be sensitive to water quality and scarcity,
requires a boost. Guided by scientific research, community participation holds the key to
mitigating and preventing As threat.
7. Conclusion
Arsenic contamination in northeastern India has a sizeable spatial spread, with many sites
exceeding the values almost 100 times the permissible limit. Fe contamination mimics a
similar pattern spatially. As and Fe co-contaminations have been found in the alluvial plains
of the Bengal Delta. Attributing the source of this contamination is of paramount importance.
Owing to the natural settings of the Himalayan foothills, heavy rainfall, population density,
biodiversity hotspots, food-habits, heavily irrigation dependent agriculture, underlying
geological features, soil-type, and microbial active dense forest coverage, the north-eastern
India provides a unique environment for comprehensive exploration of As contamination in
the groundwater. Geomorphology, plate tectonics, and rock-groundwater interaction
governed by redox processes offer critical insights into understanding the dynamics of the As
mobilization. It is imperative to control the geo-morphology and redox processes to manage
the As contamination in the region. Even though the effect of anthropogenic activities has not
been explored, geogenic contamination is a governing factor controlled by hydro-geo-
morphology and redox conditions. This study will immensely help the science community and
stakeholders develop measures to reduce the As contamination in this ecologically rich
region.
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8. Acknowledgement
This work is funded by Science and Engineering Research Board (SERB), Govt. of India under
SERB-STAR grant (grant number STR/2020/000126) and SERB-POWER grant (grant number
SPG/2021/002107) awarded to Dr. Ritusmita Goswami & Dr. Manish Kumar (WTI-Project).
9. References
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Environment, 807, 151461. https://doi.org/10.1016/j.scitotenv.2021.151461T*The study deduces that
anthropogenic input has lesser effect on As occurrence and mobilization in the Brahmaputra floodplains. The
naturally reducing aquifer environment and local geology enriches As in the region. However, lesser groundwater
recharge and increased extraction and introduction of pH altering chemicals may indirectly affect the enrichment.
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Table 1: Maximum values As and Fe reported in the northeastern Indian region and local geology. Quaternary sediments are the most
dominant geological conditions, and Assam shows very high co-occurrence concentrations of As and Fe.
State
District
Maximum
reported As
Conc. (µg L-1)
Range of Iron
concentration
(mg L-1)
Average annual
rainfall (mm)
Geology
Assam
Jorhat 5, 6, 16
657
0.22-11.02
2699
Alluvial sediments of the Quaternary age
Majuli 9, 16
468
0.49-22.00
2000
Younger alluvium deposits
Dhemaji 5, 6
200
11.45-26.84
2600-3200
Alluvial sediments of Quaternary age and Piedmont
deposits
Lakhimpur 5, 6, 33
550
1.22-49.39
1227
Neogene Siwalik Group and Quaternary Alluvium
Golaghat 5, 6
200
5.17-25.29
1300
Quaternary formation followed by Tertiary/Pre-Cambrian
deposit; Alluvial formations of river deposits
Nagaon 5, 26
353
0.76-11.02
1541
Precambrian deposit. Quartenary unconsolidated old
and young alluvial sediments.
Barpeta 5
569
1.70-16.99
2051
Piedmont deposits, Quaternary age sediments.
Nalbari 5, 6
422
4.95-22.30
1904.4
Quaternary age younger alluvial sediments
Karimganj 6
300
4067
Alluvium, Dupitila and Tipam formations
Dhuburi 6
200
2363
Precambrian deposits, younger alluvial sediments
Manipur
Bishnupur 27, 28
80
0.05-3.36
1442
Upper cretaceous to Eocene age; Disang and Barail
group
Thoubal 5, 6, 7
986
0.74-4.32
1318.39
Upper cretaceous to Eocene age: Disang group; Recent
age alluvium
Imphal East 22
50
1632.4
Upper cretaceous to Eocene age: Disang and the Barail
group; Paleo-quaternary age deposits
Imphal West 22
30
1592.4
Quaternary and tertiary deposits
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Nagalan
d
Mokokchong 5
278
0.91-1.92
2500
Tertiary age semi-consolidated deposits
Mon 5
159
0.33-1.32
2000-3000
Alluvial, Tipam, Barail and Disang formations
Tripura
*5, 6
444
0.45-10.98
1881
Quaternary age and Dupitila, Tipam and Surma
formations of Upper Tertiary age
Arunachal Pradesh
Papumpare 5, 6
74
1.95
3200
Bomdila Group; Gondwana Group; Siwalik Formations;
Quartenary alluvium deposits
Dibang valley 5, 6
618
0.132-0.251
3000-5000
Lower to Middle Paleozoic
Tirap 5, 6
90
0.259
3478.5
Disang, Barail, Tipam and Alluvial Formation
Lower Sibansiri 5,
6
159
0.29-1.14
1910
Bomdila Group
West Kameng 5, 6
127
4.931
1704.8
Bomdilla Group, Tenga Formation; Older alluvium and
River, Terraces, Siwalik Group,
Gondwana Formations
East Kameng 5, 6
58
1.051
2035
Pre-Cambrian gneisses and schists, quartzite of Bomdila
Group. Gondwana sedimentary comprising quartzite,
shale and sandstone, Siwalik Group comprising
sandstone, siltstones and Recent alluvium.
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Table 2: Major geological periods and a generalized details geological formations with
respect to Arsenic Contamination in North East India[ 51, 52, 53, 54, 55, 56, 57, 70].
Geological Time
&
Characteristic
Features
Rock formations* and Vulnerabilities**
with respect to Arsenic Contamination
Quaternary Period
Dominance of Ca-Mg-
HCO3 and Ca-Mg-
SO4-Cl.
* The Brahmaputra River system aided geological recent formations
like alluvium, riverine deposits, paleochannels and terraces; as well
as the neotectonics activities related natural levees contain high
concentration of arsenic (up to 229.0 ppb).
**During the Holocene period, a warmer and wetter climate
fostered heightened weathering mediated geological formations,
intensifying chemical weathering leading to As release.
**Indus-Tsangpo suture zone and Siwalik foreland basin,
experienced enhanced weathering of serpentinites,
consequently leading to alterations in mineral compositions, and the
subsequent release of elements like As into water sources.
Tertiary Period
Biotite, magnetite,
ilmenite, olivine,
pyroxene,
amphiboles, bearing
arsenic.
*The Himalayan rocks and the Indo-Burman ranges
contributes significantly to this contaminant.
**Metamorphism and weathering mediated mineral content
alterations leading to As release into the surrounding
environment.
Cretaceous Period
Mixing of fresh water
(Ca-Mg-HCO3) with
saline water (NaCl),
where the marine
sediments released
arsenic.
*The erosion of sulfide linked to carbonaceous material have
generated iron oxyhydroxides enriched with Arsenic.
**Reduction mediated subsequent release of arsenic into the
surrounding sedimentary environment.
Carboniferous to
Jurassic Period
Organic-rich layers,
carbonaceous sediments,
coals and ferrierites within
Gondwana rocks
**Microbial activity lead redox processes acting upon the
sedimentary sequences releasing As into the freshwater
systems.
Precambrian to
Paleozoic
Formations like
granite gneisses
**The Proterozoic basement with granites releasing As owing
to mineral dissolutions in the Meghalaya Plateau, including
the Mikir Hills of the Precambrian Indian Shield.
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Figure 1: Map highlighting a) As hazard scenario in the northeastern region (NER) of India as per the
permissible limit of 10 μg L-1 and b) the extent of areas well beyond the permissible limit. It is clear from the
illustration that there is an extensive spatial extent of arsenic toxicity in NER, and the maximum values
reported are more than 100 times the permissible limit in a few regions. No area is left untouched by the As
toxicity in the Indian NER. 1c) illustrates the geology and d) iron contamination for the NER. Fe contamination
is prevalent in almost all NER except Meghalaya, Tripura and Mizoram.
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Figure 2: Microbial transformation of As in the groundwater-surface interface facilitated by As-enriched Fe,
Mn and S minerals. These are the most prevalent As mobilization mechanisms in the northeastern region of
India.
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Highlights
• Hypothesis related to As enrichment in the northeast Indian aquifer system is investigated.
• Three governing deductions for the prevailing As spread with explicit evidences are identified.
• Tropical rain led deductive dissolution of clay deposition attributed to the seasonality of As.
• As-containing quaternary sediments succored by plate tectonics influences As-release.
• Biodiversity richness driven microbial mediated weathering is a major As enriching process.
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Author contribution:
NN: Writing – review & editing, Data curation, Software, Validation.
RG: Conceptualization, Methodology, Writing – review & editing, Funding acquisition,
Supervision
DPP: Writing – review & editing, Data curation, Software, Validation.
AK, MTS, & AB: Writing – review & editing
Manish Kumar: Conceptualization, Methodology, Resources, Writing – review & editing,
Funding acquisition, Supervision
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Declarations
Ethics approval and consent to participate Not applicable.
Consent for publication All authors have read and agreed to the published
the version of the manuscript.
Competing interests The authors declare no competing interests.
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